The present invention relates to the field of controllers, and more specifically to cascaded controllers.
The purpose of cascade control has been to provide superior control of a process through continuous controlled-medium set point adjustments. As shown in Prior Art FIG. 1, "Classic" cascade control was usually defined as a control method where the output from one control module 10 was fed into the set point input of another control module 30, passing through some scaling module 20 on the way. These functions were typically implemented using a Direct Digital Control (DDC) controller. In a DDC controller, the control modules were usually PID operators, or functions and the scaling module was typically a ratio operator.
The purpose of the scaling module was to transform the output from the first PID operator (typically ranging from 0 to 100%) into a usable set point appropriate for the controlled medium (i.e., air temperature, air pressure, water temperature, etc.). A direct linear relationship was commonly used, such as that provided by a standard ratio operator.
The problem with classic cascade control was that the system installer or end-user rarely knew what values to enter into the minimum and maximum set point limit parameters (inputs 21 and 22 into the scaling module 20) to achieve optimal control of the process. In fact, installers or end users often picked values which led to poor system operation. Also, as system (process) gains change, due to different load levels, seasonal loads, etc., the optimum values of limit parameters may change.
Accordingly, it is an object of the invention to provide installers and end users of cascaded controls with systems which have fewer parameters to be entered by them leading to reduced installation time and consequently, better performance.